Handle Errors

public class LibraryException extends Exception {}

1. public class LibraryException extends Exception {} — exactly Module 13's custom exception pattern — a one-line class that lets library-specific failures be caught and handled distinctly from generic Apex exceptions.

public Checkout__c checkOutBook(Id bookId, String memberName) {
    Book__c book = [SELECT Id, Is_Checked_Out__c FROM Book__c WHERE Id = :bookId];

    if (book.Is_Checked_Out__c) {
        throw new LibraryException('This book is already checked out.');
    }

    book.Is_Checked_Out__c = true;
    update book;

    Checkout__c checkout = new Checkout__c(Book__c = bookId, Member_Name__c = memberName, Checkout_Date__c = Date.today());
    insert checkout;
    return checkout;
}

1. public Checkout__c checkOutBook(Id bookId, String memberName) { — the method.
2. Book__c book = [SELECT Id, Is_Checked_Out__c FROM Book__c WHERE Id = :bookId]; — queries the book first.
3. (blank line)
4. if (book.Is_Checked_Out__c) { — checking this before doing anything else prevents exactly the "two members borrow the same physical book" scenario the previous lesson flagged as the reason Book__c and Checkout__c must stay in sync.
5. throw new LibraryException('This book is already checked out.'); — rejects with a clear, named failure.
6. } — closes the if.
7. (blank line)
8. book.Is_Checked_Out__c = true; — only reached if the guard passed.
9. update book; — saves the change.
10. (blank line)
11. Checkout__c checkout = new Checkout__c(Book__c = bookId, Member_Name__c = memberName, Checkout_Date__c = Date.today()); — builds the checkout record.
12. insert checkout; — saves it.
13. return checkout; — hands it back.
14. } — closes the method.

public void returnBook(Id checkoutId) {
    Checkout__c checkout = [SELECT Id, Book__c, Return_Date__c FROM Checkout__c WHERE Id = :checkoutId];

    if (checkout.Return_Date__c != null) {
        throw new LibraryException('This checkout has already been returned.');
    }

    checkout.Return_Date__c = Date.today();
    update checkout;

    Book__c book = [SELECT Id FROM Book__c WHERE Id = :checkout.Book__c];
    book.Is_Checked_Out__c = false;
    update book;
}

1. public void returnBook(Id checkoutId) { — the method.
2. Checkout__c checkout = [SELECT Id, Book__c, Return_Date__c FROM Checkout__c WHERE Id = :checkoutId]; — queries the checkout, including Return_Date__c.
3. (blank line)
4. if (checkout.Return_Date__c != null) { — a Checkout__c with a Return_Date__c already set means that borrowing event is closed.
5. throw new LibraryException('This checkout has already been returned.'); — a genuine misuse, worth a clear LibraryException rather than silently double-processing the return.
6. } — closes the if.
7. (blank line)
8. checkout.Return_Date__c = Date.today(); — only reached if the guard passed.
9. update checkout; — saves the change.
10. (blank line)
11. Book__c book = [SELECT Id FROM Book__c WHERE Id = :checkout.Book__c]; — queries the related book.
12. book.Is_Checked_Out__c = false; — marks it available again.
13. update book; — saves the change.
14. } — closes the method.